1. Field of the Invention
The present invention relates to a coaxial wiring pattern structure and a process for manufacturing the same and in particular to a coaxial signal wiring pattern structure in a thick multilayered wiring board and a process for manufacturing the same.
2. Description of the Related Art
When forming a signal wiring pattern in a thick multilayered wiring board, matching of the characteristic impedance of the signal wiring pattern with that of a semiconductor device and reduction in crosstalk noise has heretofore been performed in order to enhance the signal propagation characteristics of the signal wiring pattern. A strip line structure has been generally adopted as a signal wiring pattern. The strip line structure comprises a substrate 6, a signal wiring pattern 1, a dielectric 3 and a conductor 5 or 4 as shown in FIGS. 1 and 2. In FIG. 1, the signal wiring pattern 1 is formed on the substrate 6, the dielectric 3 is formed on the substrate 6 covering the pattern 1 and an upper conductor 5 which functions as a power supply wiring layer or a grounded layer is formed above the pattern 1. In FIG. 2, the signal wiring pattern 1 is formed on the dielectric 3 and above the substrate 6, a lower conductor 4 having the above-mentioned function is formed below the pattern 1 and on the substrate 6, and the dielectric 3 is formed on the lower conductor 4; whereby the characteristic impedances are matched. Alternatively, the strip line structure comprises a signal wiring pattern 1 and upper and lower conductors 5 and 4 which function as a power supply wiring layer or a grounded layer and which are disposed above and below the signal wiring pattern 1, respectively, so that the signal wiring pattern 1 is interposed therebetween as shown in FIG. 3, if the frequency of the signal transmitting through the pattern 1 falls in a several hundred MHz band.
Since the crosstalk noise is caused by an influence of static capacity between a signal wiring and its upper and lower conductive layers or between the signal wiring and the adjacent signal wiring, the spacing between the signal wirings is widened, the film thickness of the signal wiring is made thinner and the line width of the wiring is reduced to decrease the coupling capacity. For the signal wiring pattern in which the frequency of signal exceeds several hundred MHz or the signal potential of the neiboring signal wiring is high, the upper and lower conductive layers are disposed above and below the signal wiring, respectively, and grounded wirings 7 are disposed between the signal wirings as shown in FIG. 3.
However, the above mentioned prior art technique can not satisfactorily surround the signal wiring pattern. In a frequency band exceeding several hundred MHz, therefore, an electrical signal leaks from the signal wiring pattern and crosstalk noise is generated due to influence between adjacent wiring patterns. Also, mismatching of the characteristic impedance occurs.
The wiring patterns are generally formed by a screen printing method. In case of a wiring pattern having a wiring width of about 100 .mu.m, 325 mesh stainless screens are widely used. In this case, however, it is difficult to provide a wiring having a printed film thickness which is not higher than 10 .mu.m. In a high density packaging, the pitch among the wirings is narrow so that crosstalk noise will become serious. In a frequency band exceeding several hundred MHz, low dimensional precision of the signal wiring pattern will cause mismatching of the characteristic impedances so that abnormal propagation of the signal may occur.
Moreover, Japanese Patent Application Public Disclosure No. 84053/1988 discloses a coaxial wiring structure which comprises a first substrate having a first conductor on the inside wall of a groove formed in its surface and a second conductor disposed on the surface of a dielectric filled within the groove and a second substrate having a third conductor on the inside wall of a groove formed in its surface. The second substrate is integrally superposed on the first substrate to construct a coaxial wiring with the first and third conductors as an exterior conductor and the second conductor as an interior conductor. In this prior art, the exterior conductors and the dielectric are formed within the groove which is formed in the surface of the substrate.